Nick Feamster CS 4251 Computer Networking II Spring 2008 Medium Access Control Nick Feamster CS 4251 Computer Networking II Spring 2008

Some Multiple Access Protocols CSMA/CD Token passing Wireless LAN Protocols

Random Access MAC Protocols Non-Carrier-Sense protocols: doesn’t “listen” to the channel before transmitting ALOHA Carrier-Sense protocols: senses the channel before transmitting CSMA (Carrier Sense Multiple Access): does not detect collision. CSMA/CD (Ethernet): A node “listens” before/while transmitting to determine whether a collision happens.

ALOHA Radio-based communication network Developed in 1970s at the Univ of Hawaii Basic idea: transmit when a node has data to be sent. Receiver sends ACK for data Detect collisions by timing out for ACK Recover from collision by trying after random delay Too short: large number of collisions Too long: underutilization

Ethernet MAC If line is idle (no carrier sensed) send packet immediately If line is busy (carrier sensed) wait until idle and transmit packet immediately If collision detected Stop sending and jam signal Jam signal: make sure all other transmitters are aware of collision Wait a random time (Exponential backoff), and try again

Questions How does sender detect collision? How long does it take?

Ethernet Performance Ethernets work best under light loads Utilization over 30% is considered heavy Peak throughput worse with More hosts More collisions needed to identify single sender Smaller packet sizes More frequent arbitration Longer links Collisions take longer to observe, more wasted bandwidth

Ethernet MAC Protocol

Carrier Sense Multiple Access (CSMA) Listen to medium and wait until it is free (no one else is talking) Wait a random backoff time Advantage: Simple to implement Disadvantage: Cannot recover from a collision

Wireless Interference Two transmitting stations interfere with each other at the receiver Receiver gets garbage A B C

Carrier Sense Multiple Access with Collision Detection (CSMA-CD) Procedure Listen to medium and wait until it is free Start talking, but listen to see if someone else starts talking too If collision, stop; start talking after a random backoff time Used for hub-based Ethernet Advantage: More efficient than basic CSMA Disadvantage: Requires ability to detect collisions More difficult in wireless scenario

Collision Detection in Wireless No “fate sharing” of the link High loss rates Variable channel conditions Radios are not full duplex Cannot simultaneously transmit and receive Transmit signal is stronger than received signal

Solution: Link-Layer Acknowledgments Absence of ACK from receiver signals packet loss to sender Sender interprets packet loss as being caused by collision Problem: Does not handle hidden terminal cases.

Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) Similar to CSMA but control frames are exchanged instead of data packets RTS: request to send CTS: clear to send DATA: actual packet ACK: acknowledgement

Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) Small control frames lessen the cost of collisions (when data is large) RTS + CTS provide “virtual carrier sense” protects against hidden terminal A B

Random Contention Access Slotted contention period Used by all carrier sense variants Provides random access to the channel Operation Each node selects a random backoff number Waits that number of slots monitoring the channel If channel stays idle and reaches zero then transmit If channel becomes active wait until transmission is over then start counting again

Virtual Carrier Sense Provided by RTS & CTS Prevents hidden terminal collisions Typically unnecessary RTS CTS A B C

Physical Carrier Sense Range Carrier can be sensed at lower levels than packets can be received Results in larger carrier sense range than transmission range More than double the range in NS2 802.11 simulations Long carrier sense range helps protect from interference Receive Range Carrier Sense Range

Hidden Terminal Revisited Virtual carrier sense no longer needed in this situation RTS CTS A B C Physical Carrier Sense

Physical Carrier Sense Energy detection threshold Monitors channel during “idle” times between packets to measure the noise floor Energy levels above the this noise floor by a threshold trigger carrier sense DSSS correlation threshold Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal Triggers carrier sense if the correlation peak is above a threshold More sensitive than energy detection (but only works for 802.11 transmissions) High BER disrupts transmission but not detection